This invention relates to a variable cut-off offset press system and method of operation, and more particularly to such a system which utilizes a continuous image transfer belt.
Commercial printers have used a variety of different printing techniques such as gravure printing, flexographic printing, and offset printing. Each of these techniques basically involves the step of inking a plate containing an image to be printed and transferring the ink (which now represents an image) onto a substrate such as, for example, paper, cardboard, film, and foil.
A typical single color offset printing press includes a plate cylinder, a blanket cylinder, and an impression cylinder, each rotationally mounted on the press. Four-color presses require individual print stations with plate, blanket, and impression cylinders for each color. The plate cylinder carries a printing plate having a surface defining an image to be printed. The blanket cylinder carries a printing blanket having a flexible surface which contacts the printing plate at a nip between the plate cylinder and the blanket cylinder. The substrate to be printed, typically in the form of a continuous web or cut-sheet, travels through a second nip between an impression cylinder and the blanket cylinder.
The ink, which is required to form the image on the substrate, is first applied to the plate cylinder, transferred to the blanket cylinder at the nip there between, and then transferred to the material to be printed at the second nip. Simultaneous printing on both sides of the substrate is also possible by providing another assembly formed by another plate cylinder and another blanket cylinder on the opposite side of the substrate.
The length of the surface of the blanket cylinder (or the circumference thereof), including the printing blanket, is designed to be a multiple (typically one) of the length of the surface of the plate cylinder (of the circumference thereof), including the plate. However, should a customer require a printed image falling outside of this parameter either the press operator would not be able to print such an image with the existing press, or the print job would result in a sizable portion of substrate being wasted. For example, if the blanket circumference is less than the length of the image to be printed, the press is incapable of printing the job. Conversely, if the length of the blanket is 1.5 times the length of the image to be printed, a substantial portion of the blanket (and the underlying substrate) will receive no inked image.
Press manufacturers have developed what is termed a “variable cut-off” press to address this problem. Fixed cutoff press equipment can be made obsolete by customers changing to product sizes which do not fit the present equipment. Customers are showing an increased interest in a range of sizes to be printed. This trend may be the result of the digitization of pre-press operations.
In any event, should a printer want to accommodate print runs of different lengths, the printer can equip himself with custom presses having those cut-offs (i.e., diameters or circumferences) which are most likely to be ordered. Such a strategy is quite expensive. Other alternatives have been developed. For example, presses having removable cassettes have been introduced, such as the offset printing press described in U.S. Pat. No. 5,394,798 to Simon et al. Such a press includes both the plate cylinder and the blanket cylinder of the appropriate cutoff in a removable, replaceable cassette unit. The disadvantage of this type of press is that the cost of a cassette is approximately half that of a new printing unit and requires a considerable amount of downtime in order to change the cassette and perform the necessary adjustments to adapt to a new print job.
Converting a typical commercial offset press system to variable cut-off operation further requires variable cut-off print units and folders. A recent advancement in offset printing has been the development of a gapless blanket. Traditionally, printing blankets were formed as flat sheets which were subsequently wrapped around the blanket cylinder. The blanket cylinder required a longitudinal gap where the two opposite ends of the flat sheet met so that the blanket could be properly mounted and tensioned. Such a gap caused vibration and other problems associated with high-speed printing.
Accordingly, gapless blankets have been developed, such as, for example, the blanket described in Gaffney et al, U.S. Pat. No. 5,429,048. Such gapless blankets purport to minimize the vibrations associated with high-speed printing and are essentially relatively flexible hollow members which can be fitted onto the blanket cylinder. Typically, air passages are provided in the blanket cylinder to enable air pressure to be applied to the gapless sleeve for mounting and demounting thereof.
Other variable cut-off offset presses have been developed based on the use of gapless printing blanket sleeves. For example, Guaraldi et al, U.S. Pat. No. 5,813,336, teaches a press unit having axially removable printing and blanket sleeves. In order to provide a variable cut-off capability for the press, the print and impression cylinders are provided with so-called “saddles” which have a common inner diameter for mounting, but have a variety of outer diameters to accommodate print jobs of varying lengths. The printing plate and blanket sleeves are then mounted over the “saddles” to provide for variable cut-off printing. However, the diameters of the plate and blanket sleeves must be changed in tandem. Otherwise, residual ink on the blanket sleeve will be out of registration with each succeeding rotation and may print as a ghost image onto succeeding portions of the substrate.
Another approach to variable cut-off offset printing is taught by Erbstein, U.S. Pat. No. 5,950,536. There, a printing unit is provided with a plate cylinder and variable diameter plate cylinder sleeves which can be mounted and unmounted as required. In order to accommodate the variable diameters for the printing sleeves, the printing impression cylinder is adapted to be laterally movable. However, because the print and blanket sleeve diameters may be mismatched, the press requires either a very high ink transfer ratio between the blanket and the substrate, or blanket cleaning after each rotation to avoid residual ink on the blanket from transferring onto the substrate.
Yet, another approach to variable cut-off printing is described in U.S. patent application Ser. No. 09/159,662, filed Sep. 24, 1998, entitled CONTINUOUS IMAGE TRANSFER BELT AND VARIABLE SIZE OFFSET PRINTING SYSTEM. In this system, a continuous image transfer belt having a greater circumference than the circumference of the printing plate cylinder is used to transfer inked images onto a substrate. Simply by changing the diameter of the plate cylinder, the length of the printed image may be changed. A cleaning station is provided to remove residual ink from the belt prior to re-inking the belt.
However, problems remain in the art, particularly in obtaining precise image registration with four-color printing presses. One problem continues to be that the dimensions of the substrate change from print station to print station as the water-based inks at each print station wet the paper substrate which is driven under tension. Other problems include the need to clean the blanket surface to remove residual ink in some systems. Accordingly, the need still exists in the art for a variable cut-off offset printing system which provides precise registration of the inked images and which can be readily adapted for different sized print jobs. A need also exists for a variable cut-off offset printing system which does not require blanket cleaning to remove residual ink.